The present invention relates to a matching and tuning unit (AMTU), which connects the output from a low or medium frequency high power radio transmitter to an antenna with a complex input impedance.
Most state-of-the-art high power radio transmitters which operate in the low or medium frequency bands unitize class D switching amplifiers which are much more efficient than their predecessors which operated as Class B amplifiers. This very significant improvement in efficiency has greatly reduced the size of the modem equipment, because large cooling surfaces are no longer required. As a result, these transmitters are much more susceptible than their predecessors to the effects of mismatch at their output terminals which results in reflected power being dissipated in the heat sinks of their final amplifiers. The task of establishing and maintaining an acceptable matched condition between transmitters and antennas, which operate in the low and medium frequency bands presents a uniquely difficult situation. This is due to the fact that the physical height of these antennas is typically much lower than an optimum value. This deficiency in physical height results in an antenna input impedance that is equivalent to a low value resistance in series with a high capacitive reactance. The purpose of the AMTU is to transform this complex input impedance to the purely resistive 50 ohm value required to terminate the transmitter and to compensate for weather related changes in both the capacitive reactance and the resistance of the antenna so that the transmitter remains perfectly matched at all times.
The AMTU resonates the antenna's reactance with a series connected adjustable loading coil. Sensors measure the phase angle between the voltage and the current at the input of the AMTU. A microcontroller controlled motor adjusts the inductance of the loading coil to maintain this phase angle at zero degrees.
The value of the input resistance of the AMTU is transformed to the required 50 ohm value by the combination of a ferrite cored transformer with fixed tapping positions together with a pair of tuned, air cored, mutually coupled coils. Sensors are used to measure the value of the resistance at the input to the AMTU. A microcontroller-controlled motor adjusts the value of the coupling coefficient between the mutually coupled coils to maintain an input resistance value of 50 ohms.
In addition, the antenna current is monitored and feedback to the associated transmitter is used to maintain it at a constant value.
A sensor measures the value of the radio frequency current applied to the antenna. An RS485 serial link between the microcontroller in the AMTU and a microcontroller in the associated transmitter adjusts the output power level from the transmitter in order to maintain a constant antenna current when weather related changes to the loss resistance components of the antenna's input resistance occur.
The use of “TEE” or “PI” networks to implement the necessary impedance transformation, in which inductance and capacitor values are varied to obtain a matched condition is well known in the art. These networks suffer from the drawback that the component values are inter-dependent. A change in the value of any component affects both the resistance and the phase angle of the input impedance making automatic adjustment difficult. With the present invention the component changes to correct for reactive and resistive variations are independent, making automatic tuning and matching easier to implement.
U.S. Pat. No. 5,631,611, uses a similar technique to that embodied in the present invention in that tuned, mutually coupled coils are used to adjust the resistive component of the input impedance. However U.S. Pat. No. 5,631,611 requires a large, very expensive, high power variable vacuum capacitor to tune the secondary winding of the coupled coils. In the present invention, this variable capacitor is not required. A tuned condition in the secondary circuit of the mutually coupled coils is achieved by automatic adjustment of the loading coil inductance. The present invention also improves upon U.S. Pat. No. 5,631,611 with the addition of a feedback loop to maintain the antenna current at a constant value.
The present invention also provides an ability to monitor, control and test the AMTU from the location of the transmitter. The transmitter is typical positioned at a safe distance from the near field of the antenna where high electric and magnetic fields can be dangerous to the health of maintenance personnel. This same monitor and control feature may also be made from any location via the Internet.